drm/i915: fix WaInsertDummyPushConstPs

As pointed out by Chris Harris, we are using the wrong WA name, it
should in fact be WaToEnableHwFixForPushConstHWBug, also it should be
applied from C0 onwards for both BXT and KBL.

Fixes: 7b9005cd ("drm/i915: Add WaInsertDummyPushConstP for bxt and kbl")

Cc: Chris Harris <chris.harris@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reported-by: Chris Harris <chris.harris@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470127013-29653-1-git-send-email-matthew.auld@intel.com